Climate change is bad news for almost everyone. Emphasis on almost, because believe it or not, one marine species is absolutely thriving as the Gulf warms: Bull sharks!
Get ready for some shark science as we learn why bull sharks are increasing in numbers across the Gulf and getting hungrier.
This episode was hosted by Carlyle Calhoun and Katelyn Harrop. Katelyn conducted the interview. Our theme music is by John Batiste, and our sound designer is Emily Jankowski. Carlyle Calhoun is the executive producer. Sea Change is a WWNO and WRKF production. We're part of the NPR Podcast Network and distributed by PRX.
Sea Change is made possible with major support from the Gulf Research Program of the National Academy of Sciences, Engineering, and Medicine. Sea Change is also supported by the Water Collaborative of Greater New Orleans. WWNO's Coastal Desk is supported by the Walton Family Foundation. The Meraux Foundation and the Greater New Orleans Foundation.
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TRANSCRIPT
CARLYLE: The summer is in full swing. And on the Gulf Coast, that means we are all looking for any water to cool off in. I love this time of year, but record-breaking temperatures are challenging my summer love. And it’s not just getting hotter on land, jumping in the Gulf can sometimes feel like hopping into a bathtub. And of course these increasing temperatures not only impact our own summer experiences, but the experiences of all the creatures who call the Gulf home.
I’m Carlyle Calhoun and you’re listening to Sea Change.
Today, I’ll be handing over the mic to producer Katelyn Harrop who has a story about a marine animal that’s having something of a notable response to our region’s changing climate.
Hi, Katelyn!
KATELYN: Hey, Carlyle!
CARLYLE: Ok, I’m so excited for this interview you’re bringing us today, but I’m also totally terrified because it’s about sharks. Sharks are NOT my thing. I mean, I appreciate them as amazing animals and their importance to the ecosystem, but I just do not want to see them.
KATELYN: I so hear you. Even though sharks are TOTALLY my thing, you are not alone. Every year, Chapman University in California surveys what Americans fear most, and last year the fear of sharks ranked above public speaking, financial fraud and several natural disasters. So, you are in good company when it comes to feeling less than comfortable with the idea of sharks.
And it’s not really your fault, because sharks get some VERY bad PR. For a long time they were called “dumb and dangerous,” and any shark expert will lament the detrimental impact of the movie Jaws on public shark perception.
BUT… the reality is… the risk of shark attacks is actually extremely, extremely low. The chance of any person suffering a shark-related death is less than 1 in 4 MILLION. You are more likely to die from fireworks. That’s according to the International Shark Attack Files at the Florida Museum of Natural History.
And as for the dumb part, it’s likely quite the opposite.
CARLYLE: Ok, yes. Despite my own shark baggage, I know this is totally true. I grew up visiting my grandmother on Florida’s Gulf Coast, and we would always visit the Mote Marine Laboratory, this really cool aquarium and research center. And the scientist who founded this cool place was a woman named Euginie Clark. Have you heard of her?
KATELYN: Yes. The Shark Lady!
CARLYLE: The shark lady! Back In the 50s at the Mote Marine Laboratory, she trained two lemon sharks to press a target, which resulted in them receiving food. It’s a really similar technique to the way that dogs are trained, actually. And the sharks were able to retain this conditioned response two and a half months later. Eugenie was the first scientist to train sharks, and this experiment challenged the idea that sharks were just mindless killing machines. Or as Eugenie put it, gangsters of the deep.
KATELYN: She did so much for our understanding of sharks, and all of her work in the Gulf really paved the way for the research we’re seeing now, which is more important than ever as our oceans continue to warm. Last year the Gulf was reported to be the warmest it has ever been… with surface temperatures in the 90s.
And these rapidly warming temperatures are very bad news for many Gulf species. It’s shifting the food web, forcing some species of wildlife to migrate to cooler waters, and impacting the gulf’s long-standing fishing industry.
Climate change is bad news for almost everyone. Emphasis on ALMOST, because believe it or not, one marine species is absolutely THRIVING as our oceans warm, and changing their behaviors in the process.
CARLYLE: Sharks!
KATELYN: Well not ALL sharks, but one particular species of sharks. Bull sharks in the Gulf are doing just fine. And I talked to Lindsay Mullins, a PhD candidate in the Wildlife, Fisheries, & Aquaculture department at Mississippi State and a Research Assistant with the Northern Gulf Institute to learn all about what’s going on. She studies the ways in which our warming Gulf is changing the lives of bull sharks. She's the lead author of a paper all about it, which was published in Scientific Reports last year.
CARLYLE: Sounds fascinating. I’ll let you take it from here.
After the break, Katelyn Harrop talks with ecologist Lindsay Mullins.
INTERVIEW
KATEYLN: Lindsay, thanks so much for being here today.
LINDSAY: Of course, I'm so excited to be here and to chat with you about my favorite topic in the world.
KATELYN: I love that because I, I am hooked. I'm hooked on the story. So, Lindsay, like we said, you study sharks in Mississippi, Alabama, and across the Gulf, including the bull shark, the shark having something of a come up in the face of climate change.
But before we get into that more deeply, I wanna ask you, how is climate change impacting shark species in the Gulf more broadly?
LINDSAY: NOAA currently conducts a climate vulnerability assessment for different species of, you know, fish and invertebrates by region. So there's pretty high exposure to the effects of climate change in the Gulf.
You know, we have a lot of developed shoreline. We know that warming waters are increasing. So in the Gulf of Mexico we have a really diverse assemblage of sharks. You know, we have, um, hammerheads, we have tiger sharks, um, we have smaller, you know, black tip sharks and what they found the name of the game is that it's as diverse as the group of sharks is. There's a diversity in their responses as well.
KATELYN: Hmm. Okay. So some sharks are maybe adapting a little more easily than others?
LINDSAY: Mm-hmm. Yes, exactly.
KATELYN: Okay. And we have one shark species. That's a pretty unique example of it. It's something you've been studying really heavily, the bull shark. Um, I personally have never met a bull shark. I know you've met many. So introduce me. What do they look like? What are they into? What makes them special?
LINDSAY: Bull sharks are really, really cool sharks. They have a little bit of a bad reputation. You know, they're named bull because they have that nice blunt snout, but also they've been described as having a bit of a pugnacious personality. They're incredible predators, and one of my favorite fun facts about them is you know, we hear all about white sharks and hammerheads, you know, who both deserve love and respect, but when you compare them pound for pound, the jaw strength of a bull shark is greater than those other kinds of notorious large shark species.
Also of over 500 identified species of sharks. They're one of just a few shark species identified as Yuri hailing species, which means that they can live in both fresh and salt water. And in as far as you know, the Gulf is concerned. They have been found up as far up the Mississippi River as Illinois before.
KATELYN: Wow. Okay. So the bull shark has the really rare ability to live in both fresh and salt water, which blows my mind. Does this flexibility offer a bull shark any unique opportunities that other sharks might not be able to take advantage of?
LINDSAY: Yeah, so Bull Sharks are known to utilize nursery habitat in these kinds of estuarine, um, near shore, um, or freshwater environments near river mouths. And what it does is, you know, we think of these bull sharks as these great apex predators, but these newborn sharks can be around two feet long. And so actually when they're juveniles, it's a unique time in their life history where they can actually be vulnerable to predation. So by being able to occupy this more fresh water type habitat in these nearshore areas, not only is it a great place to find food, but also for those larger sharks that might eat them further offshore. It provides them some protection from those larger sharks who may not be able to tolerate such fresh water conditions.
KATELYN: Okay, so walk me through this. Adult bull sharks swim up from the Gulf, and that's where they give birth. How, how long do they hang around for before they head back out to sea?
LINDSAY: That's a really great question, and one that we don't know for sure. We know that they come into these nearshore areas. Usually, like I said, you know, kind of near river mouths, these bays where, you know, they'll have access to the marine water and they'll have their pups in kind of late spring, early summer, typically. And then moms aren't too involved in parental care after that, besides giving them the advantage of taking them to these near shore areas.
KATLEYN: Okay, so just to make sure I have this right, we have these sharks that are coming up from the ocean into primarily estuaries. Pretty near the shore and that's where they're having their babies about this time of year.
LINDSAY: Yeah, exactly right about now. If you were hanging out in Mobile Bay, Alabama and you went fishing, odds are pretty good. You might be able to catch a newborn bull shark.
KATEYN: Hmm. Wow. Okay. We will come back to that. But Lindsay, you co-authored this 2024 paper with experts from Alabama, Florida, Louisiana. Some very noted marine researchers in the Gulf, and you measured how the symptoms of climate change, namely warming, ocean temperatures, are impacting bull sharks, particularly their distribution and birth rates. What did you find?
LINDSAY: Yeah, so for part of my dissertation work, I was able to collaborate with the Department of National Resources in Alabama, who has been conducting this gill net survey of the entire Alabama coast for over 20 years now. Which as an ecologist is such an exciting thing because to have a kind of survey like that's conducted consistently over a long period is what enables us to make these kinds of climate change enabled observations. Um, so I took all of this catch data and coupled it with environmental data. And was able to build models that identified what the ideal habitat conditions were for these juvenile bull sharks. What was really interesting is, you know, we looked at things like dissolved oxygen. We looked at salinity, which is a very common predictor for bull sharks, but we also looked at things like chlorophyll A, which can be used as a representative basically for like prey availability and other sort of habitat change metrics, such as like, changes in land use and land cover in the adjacent shore. So what we found were really, um, three big things. So we found that there was a five times increase in the relative abundance of these Bull sharks over two decades. So you know, there's a little inter-annual variability, but by and large, we're seeing increases in their abundance.
We saw a significant increase in the amount of suitable habitat, so basically that shows us that waters are warmer and they're warmer over a larger area, and so there's more suitable habitat on average now than there was 20 years ago for these juvenile bull sharks. And what we also saw was that there was no upper thermal limit that we observed, which shows us that these juvenile bull sharks are quite robust to these warmer water temperatures.
KATELYN: Okay, so what I'm hearing you say is more bull sharks, more bull sharks in warmer waters. They're loving these warmer waters and that we don't know that there's a level of warmth that could impact them negatively. Am I understanding that correctly?
LINDSAY: Yeah, at this point there hasn't really been an observed upper limit, so we were able to observe water temperatures up to 34.7 degrees Celsius, which is around 94 or 95 degrees Fahrenheit.
And there's actually one study in the southern portion of Florida where juvenile, um, bull sharks were observed up to 99 degrees Fahrenheit. So. It's a great adaptation for these bull sharks to be resilient to such warm waters 'cause it enables them to, you know, be able to stay and, um, occupy these nursery habitats near shore.
KATELYN: I wanna zoom out for a minute here and talk about our warming oceans. What sort of temperature increase are we seeing and what does this mean for our ocean ecosystem in the Gulf more broadly outside of just the bull sharks?
LINDSAY: So on average, the Gulf is warming at about 0.05 degrees Celsius per year, and I think by the end of the century they're saying it could be as much as about three degrees Celsius change. So I think the greatest thing to realize and that even these bull sharks are kind of showing us as their habitat use change is that the status quo, or rather the dynamics of the ecosystem will be shifting throughout this time.
KATELYN: There are a lot of species who are not adapting in the way these bull sharks are.
LINDSAY: Yeah, and it's, and it's interesting too because the United States is able to boast, um, a really successful history of shark recovery. You know, since the nineties was when the first shark fishery management plan was enacted and prior to that, you know, shark populations, their landings had been declining on an order of magnitude.
So it's interesting to be in a time such as this, where you know, climate change is shuffling things around and it's happening concurrently with, you know, having seen the successful recovery of many populations.
KATELYN: Okay, so you're telling me that as our oceans warm, which is broadly a pretty startling statistic, bull sharks, particularly baby bull sharks, are actually totally thriving and they're living their best lives. How is that possible?
LINDSAY: Yeah, it's interesting because these warmer waters are still not without their drawbacks for bull sharks. There's an interesting study on the physiology of juvenile bull sharks, and on the positive it showed that these bull sharks had an increased growth rate in warmer waters, which you could hypothesize means, okay, maybe they're not as readily eaten or predated upon, or maybe that helps them secure prey more efficiently.
But at the same time, these warming waters go hand in hand with increased feeding demands and aerobic metabolism for these bull sharks. So that means they need to feed more. And that can be kind of stressful to them because it means they have to take more risks to be feeding. Because just as much as they're trying to eat to grow during this nursery time, they're also trying to avoid being eaten by something larger.
KATELYN: So they're more stressed out and they're hungrier.
LINDSAY: Yes. So they could, you know, it's definitely not impossible that they're feeling this stress, but they're also confined to the advantages of this nursery habitat.
KATELYN: Got it. Are we seeing these patterns anywhere else outside of the Gulf? Because I know there are other places that bull sharks call home.
LINDSAY: Yeah, bull sharks are great 'cause they're pretty cosmopolitan species. Anywhere that there's, you know, tropical subtropical. Waters around the world, you can usually find a bull shark near the coast and the rest of the Gulf is no exception. I had the incredible opportunity to collaborate with some professors out of Texas a and m and they've been running a survey along their Texas space for a very long time.
So my study looked at two decades. They had a four decade long study and they found an eightfold increase over those four decades. Um, and what they're able to show out of Texas is that with the waters remaining warmer for longer, that the bull sharks were also residing in the bays for longer.
KATELYN: So if we're thinking about a, you know, five to eightfold increase in the relative population of juvenile bull sharks, and they're sticking around longer, that, that just means there could be a, a really radical increase in bull sharks in a really concentrated area. Is that right?
LINDSAY: Yeah. Yeah.
KATELYN: Okay. And I understand there's also bull sharks as far as Australia, is that right?
LINDSAY: Yeah.
KATELYN: I saw this headline, I can't remember maybe a year ago now, about a bull shark in a golf course pond. Is that right? In Australia?
LINDSAY: Yes. This is a true story. There are actually many bull sharks in this pond at one point. So there was a golf course in Australia that had some river waters nearby and they underwent a flooding event. And from that flooding event, bull sharks were able to get caught in the golf course pond. And not only were they caught in the pond, but they were obviously remained in the pond for up to 17 years, I believe it was. Oh my gosh. And the really interesting thing for me as an ecologist is not only that they persisted in the water that long, but it was also observed that, you know, juveniles were able to reach maturity within the pond.
And like the only reason that, you know, we say, okay, up to 17 years. Is not because there was some physiological limit of the bull shark at that point. It's just that another flooding event had occurred and the bull sharks, you know, got back into the river system essentially. But I love that story too, because the golf course really embraced it. Like they rebranded to make the golf course mascot a bull shark, and they had a cute little sign next to it, like warning sharks in the pond. So. I don't know, I wouldn't go after the ball if it went into that pond, I knew would be a pretty, pretty cool, uh, thing to have seen for sure.
KATLEYN: Yes, it's a great story. Something of a draw. So, Lindsay, when I think about the findings of your study, you know, an increase in relative abundance of bull sharks as our oceans warm. And the fact that they're able to exist in these habitats, in these estuaries for longer, with no question about how warm these waters can get at this point.
I feel like these findings are emblematic of a really interesting truth about climate change that can be kind of tricky to talk about, which is that while most of us, many of us are suffering under climate change, there are winners too, right.
LINDSAY: Yeah. Yeah. It's funny because this story seems sort of contrary to the general narrative that we hear about climate change and what we're learning is that it can be a bit more nuanced to that and that it really requires a very species to species approach, um, with regards to management style.
So, you know, I like you said, I'm not saying that climate change doesn't pose a major threat to our oceans, but our approaches can't be one size fits all. You know, for example, we see all these instances where corals have exceeded their thermal maximum, and as a result, you know, have undergone coral bleaching.
But at least in the instance of the Gulf Bull Sharks, it's not the thermal maximum that seems like it's posing any kind of limit yet.
KATELYN: But can this last, you know, if, if we're looking at this, this ongoing trajectory of, of warming oceans, and not just warming oceans, but increased storm events and rising sea levels, could there be a point where bull sharks are no longer able to evolve to this changing ecosystem?
LINDSAY: Yeah, that's a really important question and one I definitely want to underscore is that no, it's not impossible that there couldn't be a thermal maximum reach for these bull sharks. It could also mean that the thermal stress is not what is going to be more stressful to them. It could be things such as a shift in prey availability.You know, if there's another piece of the ecosystem that goes out of whack, you know, it could certainly affect the bull sharks.
KATLEYN: Mm, okay. Yeah, and we talked about this a little bit already, but we know that bull sharks are reproducing and they're exhibiting these unique distribution patterns, but at the risk of getting sort of existential here, are the bull sharks actually doing well? Are they actually thriving instead of just surviving? Or do these warming conditions also pose some challenges to their wellbeing and, and sort of how they exist right now?
LINDSAY: Yeah, so that's one thing that I've kind of thought about with these models is, okay, so we see this increased relative abundance and you know, that is indicative to us of, okay, they have the capacity to be reproducing right now.
Because normally that is a very energetically demanding thing. And so that to me is still a good sign. But what we do know is that, like I said, these mature bull sharks will move offshore to regulate their own temperatures. Hmm. And so the thought is that maybe these juvenile bull sharks. You know, they have to be more resilient under these warmer waters because they need to make sure they're not going to be eaten by a larger shark, and they need to have this kind of prey availability.
[00:21:18] They're having to kind of make a trade there of accepting this metabolic stress that can be induced by these warmer waters in exchange for the protections that a near shore nursery habitat will provide them.
KATELYN: In 2020, the International Union for Conservation of Nature and Natural Resources, the IUCN listed the bull shark as vulnerable on their red list of threatened species. That's the level right before endangered and accounts for bull sharks worldwide, not just in the Gulf. Are there other gulf shark species who get that vulnerable or even close to endangered rankings? Are there other species we're watching?
LINDSAY: Yeah, so in the Gulf of Mexico there were, um, several species that people would probably be familiar with that were identified as having really high vulnerability to climate change. There was the scallop hammerhead and the great hammerhead, and both of those are critically endangered according to the IUCN. So their populations are decreasing. Another one that was identified as high risk and is also protected in their endangered species Act. It's technically a ray, but we'll include it.
It's a small tooth sawfish, and historically they were found throughout the Gulf of Mexico, but today they're only found off the coast of Florida, and that's mostly due to habitat loss and overfishing.
KATELYN: So it sounds like even though bull sharks are adapting really well, there are a lot of shark species who are very concerned about, as these waters warm.
LINDSAY: Yeah, yeah, definitely wants to watch for. Another really interesting kind of charismatic species people love is the tiger shark, and at least with regards to the east coast of the United States, there was a 2022 paper that came out that showed that tiger shark migrations have extended poleward. And there are arrival times in these northern latitudes. They're happening earlier in the year concurrently with higher sea temperatures. And also the densities of tiger shark catches over the last 40 years have moved poleward, so it's a higher concentration. They're arriving more northward and they're arriving earlier in the year.
KATELYN: I wanna talk about that food web that you mentioned. Sharks are often considered an apex predator at the juvenile stage for bull sharks, maybe a little bit more of a middle tier predator. I wanna talk about what the rise of bull sharks could mean for our coastal ecosystem more broadly downstream, especially when we think about fish who come below them on the food chain.
LINDSAY: That's a really good question, and one thing that I think a lot of people are asking right now, because you know, like you said, not everyone loves sharks and there's reports of these increasing numbers and people are saying, Hey, there's too many sharks in the ocean. This is actually gonna be bad. But the truth is, you know, we don't have data that truly inform us of what the baseline levels for these sharks populations in the Gulf were prior to being completely decimated by, you know, overfishing.
So we're still kind of in the observing phase of, um, seeing what is that healthy baseline, what are these thresholds and what should appropriate population levels be.
KATELYN: Lindsay, I wanna ask you a question I love to ask ecologists, which is, when you think about your species, the bull shark in this case, what can their reality tell us about our ocean system more broadly and about the impact of climate change on the ocean system that these bull sharks call home?
LINDSAY: Sharks are commonly referred to as indicator species. And one of the things, you know, ways you can think about it is if you see a shark in the ocean, sort of contrary to the shark infested waters, which sounds like a negative thing, the presence of a shark is indicative of a healthier ecosystem. So for these sharks at the upper level of the food web, it tells us, Hey, there's enough going on underneath that can still support them to have these healthy populations. 'cause a lot of times when a food web is disrupted, you'll kind of see those upper level predators will be some of the first indicators to go. So sharks are an indication of a good, healthy habitat.
KATELYN: So Lindsay, this is kind of the elephant in the room here. You're telling me we have an increase in abundance of bull sharks. They're staying in estuaries longer, they're loving the warmer water. They're close to shore at the juvenile stage. What does this mean for us? Because what I'm hearing is more bull sharks riled up by warming waters unafraid to be near shore.
Should we be worried about human safety?
LINDSAY: Yeah, that's a really common question when it comes to sharks, you know, I wanna underscore that the probability of being bitten by a shark is extremely, extremely low. When we're talking about this fivefold increase in abundance, I'm talking about juvenile bull sharks, and I think a lot of times people have in their mind that like, oh, it's this, you know, insane concentration of 11 foot bull sharks near the shore, and mature bull sharks can come near the shore, but in terms of the observed population increases that we've seen, I can only speak to the juvenile bull sharks and those are, you know, two to three feet long and are much more worried about being predated than they are about being interested in you.
KATELYN: So maybe we don't need to be too worried about a bull shark bite or more worried that we need to be about any other species. But how about when it comes to the Gulf's fishing industry? That's a huge part of the local economy. Does the increase in baby bull sharks spell trouble for anglers?
LINDSAY: So sharks are really, really smart. And so as far as fish go, the brain size to body size for sharks is actually relatively high. And so what we do know is they can exhibit a learned behavior where they begin to associate boats and things with food.
And so one of the common complaints we're seeing now with recovered shark populations in the Gulf is an issue pertaining to something called depredation, which is when a fisherman's catch is either completely or partially removed from the line. So you think you're out fishing and you're excited about what you've caught, and a hungry shark comes by and sees free food, and it's like, well, yeah, I'm gonna take some of that. Takes a bite off your line. And now for the commercial fishermen that can represent an economic loss. They're hungry and they're near shore and they're smart.
KATELYN: A lot of your work focuses on conservation, so tell me why is it so important to protect not only bull sharks, but all sharks who live in the Gulf.
LINDSAY: Hmm. You know, we have such an incredible diversity of sharks here, and because they're highly mobile species, they regulate the prey populations beneath them. They also mitigate you know, negative behaviors of their prey fish. So for example, there's a research study in which they looked at different seagrass habitats, um, that did or didn't have tiger sharks.
And what they found is that because of the kind of intimidation factor of having a tiger shark in a seagrass habitat, their prey species such as the turtle, were able to over-graze on these really important primary producers, the sea grass. Um, and so by actually having bull sharks in a habitat, they help not only keep their prey’s numbers in, in balance, in check, but they also are keeping, you know, the rest of the system, keeping them behaviorally in check.
KATELYN: Lindsay Mullins is a shark ecologist with a focus on juvenile bull sharks. Lindsay, thank you so much for taking the time to talk to me today. I really appreciate it.
LINDSAY: Yeah, you're so welcome. Thank you for your interest in the topic. I'm so happy to share about it whenever I can.
OUTRO
Thanks for listening to Sea Change. This episode was hosted by me, Carlyle Calhoun and Katelyn Harrop. Kaitlyn conducted the interview. Our theme music is by John Batiste. Our sound designer is Emily Jankowski and I'm the executive producer. Sea Change is a WWNO and WRKF production. We're part of the NPR Podcast Network and distributed by PRX.
Sea Change is made possible with major support from the Gulf Research Program of the National Academy of Sciences Engineering and Medicine. Sea Change is also supported by the water collaborative of Greater New Orleans. WWNO'S Coastal Desk is supported by the Walton Family Foundation. The Meraux Foundation and the Greater New Orleans Foundation. Stay cool. We'll be back in two weeks.
